Controlled Plasma Nitriding for an Optimum Balance between PACVD TiBN Coating Scratch Resistance and Substrate Toughness

Abstract

The plasma nitriding (PN) process in the duplex surface treatment was controlled to create nitrided diffusion layers with depths of 0, 5, 15 and 80 m in the substrate of the DIN 1.2367 hot-work tool steel with the maximum microhardness values of 600, 700, 820 and 1000 HV, respectively. The scratch properties, i.e. the critical loads of cohesion (LC1), adhesion (LC2), breakthrough (LC3) and worn out (LC4), of the PACVD TiBN coating (boride, 5-7 at.%) on these substrates increased linearly with the maximum hardness of the PN diffusion layer. Instead of the composite hardness, the peak scratch hardness was used to describe the load-carrying capacity of the TiBN coating and PN substrate. Deep tensile cracks in the PN substrate with a hardness value of 1000 HV formed during the scratch test at a load as low as 90 N, indicating the low fracture toughness of the substrate. Therefore, an optimum balance between the scratch properties of the coating and the good fracture toughness of the nitrided substrate must be achieved through exercising the control of the PN and PACVD duplex process.